Add Mesen to the built-in list of emulators

[Former user]

Mesen is a a heavily developed NES/Famicom emulator that is very stable and accurate: https://github.com/SourMesen/Mesen . Besides its superior emulation capabilities, it also automatically saves game states by default when you close the game, and reloads the saves when you reopen the ROMs.

FCEUX uses old Windows controls and often hangs on Windows 10 when you close it from the fullscreen mode, and work on Nestopia ceased many years ago.